Whenever a blackhole is perturbed with a probeThe probe field is not heavy enough to change the black hole global temperature. the perturbation propagates with certain discrete frequencies due to fixed boundary conditions at horizon and asymptotic infinity[1]}. The flux has to be ingoing at the blackhole horizon and outgoing at the asymptotic infinity. The frequencies depend upon the parameters of the probe and the black hole. For asymptotically flat space-time there have been many works in the gravitational context starting from the 1970s[2]}, [3]}, [4]}. The calculation of the quasinormal modes (QNM) follows from solving the field wave equations of the probe in the black hole background with suitable boundary conditions, and then calculating the emission/absorption cross-section using classical scattering theory.
